It's been brought up in a couple articles that the real reason for Harrington's unhappiness stems from his contract. His agent is Dan Fegan. There is no such thing as a Scott Boras of the NBA (the CBA prevents that from happening), but Fegan is basically as close to a Boras as anyone in the NBA. He's telling Al to look out for his next contract.
He has 2 years left (including this year). Same with Stephen Jackson. He sees the team working on an extension for Jackson, but not him. So basically (as many of us have known since the Warriors traded for him), the Warriors don't view Harrington as part of the long term future.
Harrington sees himself turning 30 in two years. He probably wants to go to a place where someone will give him an extension immediately or will give him the playing time to prove that he's worth another big multi-year deal.